In recent years, digitalization of cordless telephones used in houses has been in progress, and high-sound quality and improvements in the security of cordless telephones have been made.
When voice or video communication or data is made shareable by building a link between a cordless telephone disposed inside a house and a mobile telephone, the base unit of the cordless telephone and the mobile telephone need to be synchronized with each other so as to be capable of communicating with each other at any time while the mobile telephone is placed in the house. However, when the mobile telephone is taken out to the outside of the house, the communication function of the mobile telephone for communicating with the base unit need not be operated. Even when various communication functions are built in the mobile telephone, putting these functions in operation always causes an increase in power consumption.
For example, Patent Literature 1 discloses a wireless telephone apparatus that avoids increases in power consumption in the case of being out of a service range. According to the disclosure, when it is detected that the wireless telephone apparatus is out of a service range in which electric waves transmitted from the base unit in the house are receivable and when this state of the wireless telephone apparatus continues at least for a predetermined time, the wireless telephone apparatus stops searching for the base unit (open search), thereby avoiding increases in the power consumption when the wireless telephone apparatus is out of the service range. In addition, it is possible to resume searching for the base unit as appropriate by being triggered upon an operation performed in an operation section or the like and to return the wireless telephone apparatus to the communicable state.
Patent Literature 2 discloses, as a system capable of detecting the location of a portable handset without using a GPS, a location detecting system in which a plurality of base units performing short range communication are arranged in a lattice pattern at intervals of a maximum communication distance, so that a portable handset can detect the location of the portable handset by searching for the base units always. According to this location detecting system, a search procedure is repeated in which: the portable handset transmits the ID thereof at the time of finding a base unit; a link is built through authentication given by the base unit; the link is released after the building of the link; and a search for another base unit is performed. Each of the base units herein receives the ID of the portable handset and transmits the received ID to a host computer in this system. Since the location of each base unit is known and is recorded in the host computer, the location of the portable handset can be identified based on the location of the base unit that has communicated with the portable handset having the corresponding ID.